"in the red " is not a real accurate term. The gauges will tell you generally that something might be wrong, if that reading is different than what it normally is. What you should do is If the coolant has not been replaced in the last few years, then drain it out, flush the radiator and engine , if the thermostat has not been changed in some time, now is the time to do that also.The radiator should also should be blown out of all bugs and debris.Refill with proper mixture coolant , pressure test cooling system for leaks, also test radiator cap.This will fix most cooling system problems, unless the radiator is becoming clogged and not allowing coolant to flow, or water pump or head gasket problem.If you have oil in the coolant or coolant in the oil, do a compression test.Depending on how good you are with a few tools you can do the first part yourself. Getting into the engine will demand a little more mechanical knowhow but there are some real good hands on guys on this site that can walk you thru .Hopefully it will not need that, just a little TLC.
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